1. Field of the Invention
The present invention relates to a method of controlling a nuclear reactor during a transient period. The method includes actuating the steam dump (bypass) system in response to a temperature error signal and a power mismatch signal.
2. Description of the Prior Art
In commercial PWRs that are utilized to generate electrical power, reactor coolant water (or primary water) recirculates between a reactor pressure vessel and one of a plurality of in-parallel steam generators in a closed loop known as a reactor coolant system (or a primary system). In a steam generator, the heat in the recirculating primary water flowing through the primary side (i.e., the tube side) passes through the walls of the tubes and is absorbed by relatively cool secondary water flowing on the secondary side (or shell side). The transferred heat generates steam on the secondary side at a temperature of about 500° F. or more and at a pressure of about 800 psi or more. The steam flows out of the steam generators to turbines that generate the electrical power. The exhaust steam from the turbines is condensed and recirculated to the steam generators as feedwater.
An increase in reactor power can increase the rate of heat transfer to the reactor coolant water which can increase the rate of heat transfer to the secondary water causing more steam supplied to the turbine for transformation into electrical power. Conversely, if less electrical power is required, the power requirement of the turbine diminishes. The steam flow to the turbine is reduced and the turbine utilizes less of the steam being transferred to the secondary water. Since less steam is being drawn from the secondary side when the steam flow is reduced, both the temperature and pressure of the steam generator secondary side can increase. The effect of this increase in secondary water temperature is reflected in the reactor coolant water since less heat can be transferred from the primary water to the secondary water in the steam generator. As a result, both the temperature and pressure of the reactor coolant water can increase.
A decrease in turbine power over a period of time is referred to in the art as a load rejection. If the load rejection is such that the reactor regulating systems, like the rod control system and the steam dump system, are unable to compensate rapidly enough for the change in power and, the temperature and pressure of the primary water increases uncontrollably, protective systems come into operation to trip the reactor and/or to open steam safety valves to avoid an overpressurization in the primary and secondary systems.
The steam dump valves operate in conjunction with the turbine and the reactor to enable the prevention of excessive pressures in the primary and secondary systems, thereby allowing the reactor to stay operational in a partial or even a complete load rejection transient. The steam dump valves operate to remove excess steam from the system. The steam dump valves can be actuated when the reactor coolant average temperature (Tavg) exceeds an established setpoint or reference temperature.
A load rejection can be initiated by the operator or by an automatic signal. A 50% load rejection is a design basis requirement for commercial PWRs. In this situation, the turbine power is reduced from 100% power to 50% power; and the nuclear power, i.e., the power generated by the reactor pressure vessel, initially remains at 100%. Since the nuclear power is greater than the turbine power, the reactor coolant average temperature and pressure will increase. The rod control system will insert the rods to reduce the nuclear power; however, it will take some time to reduce the nuclear power. Thus, opening of the steam dump valves can quickly dissipate the additional nuclear power thereby slowing or precluding the increases in reactor coolant water temperature and pressure.
There are some nuclear plants that have implemented variable temperature operation at 100% power which means that the plants are operating at a reactor coolant average temperature (Tavg) that is lower than the typical value. Operation at a lower Tavg can reduce the steam dump capacity and thus, can limit the capability of a plant to sustain load rejection transients. When plants are operating at a lower Tavg, the nominal steam pressure is lower. This lower steam pressure can reduce the steam dump capacity at early stages of the transient. Currently, this may be addressed by revising the deadband and proportional band of the steam dump controller. One disadvantage to this solution is that it introduces primary and secondary side parameter fluctuations.
Thus, there is room for improvement in the art to provide a method of controlling a nuclear reactor during a transient, the nuclear reactor being operated at a lower reactor coolant average temperature, while maintaining the capacity of the steam dump system to provide rapid and early relief to avoid a reactor trip in order to improve plant operability.